1. Field of the Invention
The disclosures herein generally relate to memory devices, and particularly relate to a nonvolatile memory device.
2. Description of the Related Art
Nonvolatile semiconductor memory devices include flash EEPROMs employing a floating gate structure, FeRAMs employing a ferroelectric film, MRAMs employing a ferromagnetic film, etc. In the case of EEPROMs, there is a need to manufacture a transistor having a special structure comprised of a floating gate. In the case of FeRAMs and MRAMs, which achieve nonvolatile storage by use of a ferroelectric material and a ferromagnetic material, respectively, there is a need to form and process a film made of these respective materials. The need for such a transistor having a special structure and the need for such a film made of a special material are one of the factors that result in an increase in the manufacturing costs.
There is another type of nonvolatile semiconductor memory device, which achieves a nonvolatile data storage function by utilizing an MIS (metal-insulating film-semiconductor) transistor as a memory cell transistor. This MIS transistor has the same structure as ordinary MIS transistors used for conventional transistor functions (e.g., switching function). Namely, this memory cell transistor requires neither a special structure such as a floating gate nor a special material such as a ferroelectric material or a ferromagnetic material. Provision is made such that the MIS transistor utilized as a memory cell transistor experiences a hot-carrier effect on purpose for storage of one-bit data. Here, the hot-carrier effect refers to the injection of electrons into the oxide film. The presence of electrons trapped in the oxide film causes a rise in the threshold voltage, which causes the drain current to be decreased. The amount of drain current of a given MIS transistor may be measured to determine whether this transistor has electrons trapped in the oxide film. The presence of trapped electrons may represent one-bit data “0”, for example, and the absence of electrons trapped in the oxide film may represent one-bit data “1”, for example.
In order to determine whether a given transistor has electrons trapped in the oxide film, the drain current of the transistor may be compared with a reference current. A drain current smaller than the reference current indicates that there are electrons trapped in the oxide film. However, different MIS transistors have different threshold voltages due to variation in the number of impurities contained in the semiconductor substrate. Because of this, the amount of drain current varies from transistor to transistor. Such variation in the amount of drain current may cause an error in the detection of a stored value. Namely, a transistor having a relatively low threshold voltage from the beginning may be likely to have a relatively low threshold voltage after injection of electrons. Since the drain current of such a transistor is relatively large, the drain current may exceed the reference current, resulting in erroneous data detection.